Fault isolation of an optical link by correlating PMD events with other measurements

ABSTRACT

A method and apparatus performing fault management in an optical communications system including a polarization mode dispersion (PMD) compensator. The method and apparatus utilizes a controller receiving notifications indicative of PMD in the system, as well as at least one indicator from a system device providing an indication of an attribute of the communication system. Such system devices include a receiver providing a Q factor and a bit error rate (BER) of an optical signal, and a signal-to-noise ratio meter providing an SNR of the optical signal. The method intelligently provides fault management by correlating the PMD notifications and system indicators to distinguish between fiber failure, PMD-related degradations, and failure of monitoring equipment itself.

CROSS REFERENCE TO RELATED APPLICATIONS

Cross reference is made to co-pending U.S. patent application Ser. No.09/220,732 filed Dec. 24, 1998 entitled, “Method of Limiting PMD in anOptical Communications Link” which is assigned to the same assignee asthe present invention and the teachings of which are included herein byreference.

FIELD OF THE INVENTION

The present invention relates to optical communication networks, andmore particularly to a method and apparatus monitoring the quality of anoptical path having polarization-mode dispersion (PMD) to intelligentlyimprove the quality of the optical path.

BACKGROUND OF THE INVENTION

In a typical optical communications system, an optical signal in theform of a series of light pulses is emitted from a modulated opticaltransmitter comprising a laser diode. Each light pulse is of extremelyshort duration, such as 40 ps, and is roughly Gaussian shaped as afunction of time. In the frequency domain, this signal comprisesnumerous frequency components spaced very closely about the nominalcenter frequency of the optical carrier such as 193,000 GHz. As thistype of modulated optical signal passes through an optical fiber,different frequency components of the optical signal travel at slightlydifferent speeds due to an effect known as chromatic dispersion. In thecourse of an optical signal traveling through a very long fiber, such as200 km, chromatic dispersion causes a single pulse of light to broadenin the time domain, and causes adjacent pulses to overlap one another,interfering with accurate reception. Fortunately, many techniques areknown for compensating for chromatic dispersion.

Another form of dispersion is becoming a limiting factor in opticalcommunications systems as progressively higher data rates are attempted.Polarization-mode dispersion (PMD) arises due to birefringence in theoptical fiber. This means that for two orthogonal directions ofpolarization, a given fiber can exhibit differing propagation speeds. Alight pulse traveling through a fiber will probably, unless some controlmeans are employed, have its energy partitioned into polarizationcomponents that travel at different speeds. As with chromaticdispersion, this speed difference causes pulse broadening and restrictsthe usable bandwidth of each optical carrier.

Schemes to actively compensate for PMD generally involve detecting thepresence of polarization-dependent timing differences and either a)applying delay elements to one or the other polarization to realign thetiming of pulses or b) controlling the polarization state of the signalupon entry into the fiber, or at intermediate points along the fiber,such that birefringent effects are minimized or canceled out. Activecompensation techniques are required because the PMD of a given fibervaries over time due to temperature and pressure changes along thefiber, and due to aging. A fiber that is installed above ground canexhibit fairly rapid fluctuations in PMD due to temperature andmechanical forces. A fiber buried underground can be sensitive to loadssuch as street traffic or construction work.

A modulated optical signal arriving at an optical receiver must be ofsufficient quality to allow the receiver to clearly distinguish theon-and-off pattern of light pulses sent by the transmitter.Conventionally, a properly designed optical link can maintain abit-error-rate (BER) of 10⁻¹³ or better. Noise, attenuation, anddispersion are a few of the impairments that can render an opticalsignal marginal or unusable at the receiver. Generally, when an opticalchannel degrades to a bit-error-rate of 10⁻⁸, a communications systemwill automatically switch to an alternate optical channel that has abetter BER.

One common method of analyzing the quality of a modulated optical signalis a so-called “eye diagram”, shown in FIG. 1. The eye diagram consistsof overlaying successive frames of time-domain traces of the signal,with each frame corresponding to one period of the nominal periodicityof the modulation. As portrayed, the vertical axis representsinstantaneous intensity of the received signal, and the horizontal axiscorresponds to time. Many successive traces of transmitted “ones” and“zeros” define a region or window within the middle of the display. Inthe time axis, the window is bound on either side by the transitionalleading and trailing edges of the pulses. Using this technique, a largeclear area or “window” in the center with no encroachment from any siderepresents a good signal in that the present or absence of a pulseduring each clock cycle is clearly distinguishable.

Noise added to a signal appears as “fuzziness” of the lines defining thewindow. Sufficient noise can even obliterate the appearance of thewindow, representing a bad signal in that “ones” and “zeros” are nolonger distinguishable. Impairments in the time axis, such as chromaticdispersion or polarization mode dispersion, cause the transitional areasof the display to close in upon the window from either side. Overlappingof pulses can require more stringent synchronization of the receiver'sdecision point, or even render the signal unusable.

A given optical receiver will automatically adapt to receive a modulatedoptical signal. Automatic gain control (AGC), frequency control, andphase lock-in are typically applied in sequence so that a thresholddecision circuit can best sample the signal and decode every pulse.Superimposed upon the eye-diagram, an optimal point of operation for athreshold decision circuit would intuitively be at the center of thewindow, as shown by the “+” in FIG. 1. This means that the intensitythreshold is about halfway between the zero values and one valuesobserved on average.

Timewise, the center of the window corresponds to sample the pulses inthe middle of their duration when they tend to be of maximum intensityalso shown by the “+” in FIG. 1. Intuitively, one can see how thischoice for an operating point would be the most robust against eithernoise or timing impairments which cause the window to shrink.

A received optical signal can undergo some degree of change in, forexample, pulse width without having an immediate impact on BER asobserved by this optimally positioned main decision circuit. Aparticular type of receiver has been developed comprising at least twoindependent decision circuits of the type just described. Reference ismade to an article entitled “Q-factor Measurement for High Speed OpticalTransmission Systems”, authored by A. J. Ramos which is from proceedingsof the SubOptic '97 conference, San Francisco Calif. 1997,891, as wellas an article entitled “Margin Measurement in Optical Amplifier Systems”authored by Bergano, Kerfort and Davidson, Photonics Technology Letters,5(1993)304, the teachings of which are incorporated herein by reference.A main decision circuit is dedicated to actual communications receptionand is maintained at the optimum point, once it is established, withinthe center of the window. But for analyzing signal quality to a finerdegree and for measuring degradation before it impacts BER of the actualcommunications, an auxiliary decision circuit is used to probe theextents of the operating window. Robustness to timewise disturbances isgaged by directing the auxiliary decision circuit to sample at varioustime offsets relative to the optimum point. Findings by the auxiliarycircuit may even be used to fine-tune the optimum decision pointsettings of the main decision circuit.

The auxiliary decision circuit is set to a given timing offset and itsoutput is monitored for BER, especially in comparison to the output ofthe main decision circuit. The BER measurement at each operating pointcan typically take several minutes. Gradually, BER data is accumulatedfor every offset value. As expected, a plot of this data resembles aninverted Gaussian curve with a minimum BER occurring some optimumoffset, as shown in FIG. 2. A similar plot is derived by varying theamplitude threshold of the auxiliary decision circuit.

All of this BER data may be summarized into a “Q” factor or qualitymetric for the received signal. In general terms, the broader the rangeof timings over which a low BER can be sustained, the greater the Qfactor of the signal. A receiver with an auxiliary decision circuit canmeasure and output such a Q factor.

The Q measurement is particularly useful for assessing and fine-tuningan optical path that is already operating at a healthy low BER. The Qmeasurement estimates a BER without requiring any actual bit errors tooccur. A Q measurement covering the BER range of 10⁻¹³ to 10⁻²⁰ may becompleted in a few minutes, whereas an actual errored bit might not beobserved for hours, days or months.

When an optical path degrades, some corrective action may be necessaryeither to improve the optical path or to divert the communicationstraffic along an alternate channel or path that will work better. Yet,it is equally important to the integrity of the traffic bearing signalto avoid taking unnecessary corrective actions. Each adjustment orswitching operation can temporarily disrupt the traffic bearing signal.

During the time that an auxiliary decision circuit is accumulatingmeasurements to compile a Q factor for a received signal, a shift indispersion characteristics, particularly PMD characteristics, can takeplace along the fiber. This can result in an inaccurate assessment ofthe signal quality, especially if a PMD compensator cannot quickly andsufficiently compensate for the PMD change. Therefore the Q factorcannot be solely relied upon as a measure of path quality.

A technique and system is required for monitoring the quality of anoptical path in an optical communications system and taking appropriateactions to either make adjustments to the path or switch to anotherpath.

SUMMARY OF THE INVENTION

The present invention achieves technical advantages as a method forproviding fault management in an optical communications system bycorrelating observations from PMD compensators with indicators from atleast one, and preferably a plurality, of other system devices. Invarious embodiments of the present invention, the system devices maycomprise an optical receiver providing a Q factor as the indicator of anoptical signal passing through the communication system. This receivermay also provide an actual observed bit error rate (BER) of the opticalsignal as another indicator. Another system device may comprise asignal-to-noise (SNR) meter providing a direct SNR of the opticalsignal.

The method of the present invention correlates notifications from a PMDcompensator indicative of PMD in the optical system with other systemindicators, such as the aforementioned indicators. By correlating thePMD notifications in view of these other indicators, a controllerreceiving the notifications and the indicators can better determine afault in the optical system and make corrective action. Several inputsare integrated and processed by the controller to distinguish between,among other things, fiber failure, PMD-related degradations, or failureof the monitoring equipment itself. The outputs provided by the methodof the present invention may be used to alter protect switching logicand to alert network maintenance personnel as to the probable cause ofdegraded path indications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an eye diagram which is typically used to analyze thequality of a modulated optical signal;

FIG. 2 is a graph of a typical BER measurement as a function of offsetwhich can be used to determine a Q factor of an optical signal;

FIG. 3 is a block diagram of an optical communications system accordingto the present invention having a controller adapted to perform themethod of the present invention;

FIG. 4 is a flow chart illustrating how the controller ascertains andprocesses several inputs from the optical communication network todistinguish between fiber failure, PMD-related degradations, and failureof monitoring equipment according to the preferred embodiment of thepresent invention; and

FIG. 5 is a flow chart illustrating how a Q measurement is calculatedand recorded as a valid recent Q measurement for use in the algorithm ofFIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 3, there is illustrated a block diagram of anoptical communication system 20 according to the present invention.System 20 includes a microprocessor based controller 22 receiving andprocessing several indicator inputs from the system to perform faultmanagement according to the preferred embodiment of the presentinvention. System 20 is seen to include an optical transmitter 24including a semiconductor laser emitting light that is intensitymodulated by a corresponding electrical data signal provided on inputline 26. The electrical data signal can be a SONET-compliant STS-48 orSTS-192 synchronous data signal bearing digital data at about 2.5 Gbpsor 9.9 Gbps, respectively. The intensity modulated optical carrier isprovided by transmitter 24 into optical fiber 28. The optical carriermay be a SONET OC-48 or OC-192 signal bearing digital data atapproximately 2.5 or 9.9 Gbps, respectively. The optical fiber 28 mayinclude an optical amplifier generally shown at 30 for amplifying theoptical carrier along the length thereof. It is noted transmitter 24 mayinclude several semiconductor lasers, each providing light that isintensity modulated by a corresponding input electrical data signal toprovide Wavelength Division Multiplexing (WDM) if desired. For purposesof teaching and illustrating the present invention, a singlesemiconductor laser generating a single optical carrier is discussed toteach and describe the present invention with it being understood thatthe present invention can apply to several or all optical carriers andbeing communicated over a common optical fiber.

Still referring to FIG. 3, optical system 20 can be seen to include apolarization mode dispersion compensator (PMDC) 32 provided alongoptical fiber 28. The PMDC 32 actively compensates thepolarization-dependent timing differences of the optical signal. For amodulated optical signal, the PMDC 32 continuously senses the timingdifference between the pair of orthogonal polarizations and selectivelydelays one polarization to realign the timing between the two signalhalves before passing the signal to a receiver. As the polarizationcharacteristics of the fiber change, the PMDC 32 constantly monitors theoptical signal and adjusts the delay to minimize the PMD contribution tooverall dispersion. PMDC 32 also provides several PMD notifications tocontroller 22 on output line 34, as will be discussed further shortly.In practice, one or several PMDCs 32 can be provided along the length ofoptical fiber 28, and can be provided at several locations includingproximate the transmitter 24 for providing forward compensation, in themiddle of the link, proximate the receiver, or any combination thereofdepending on the design of the optical communication system. Thus,limitation to the location or number of PMDC 32 is not to be inferred inthe present invention.

Optical system 20 is further seen to include a signal-to-noise ratio(SNR) meter 36 receiving a tapped a portion of the optical carrier froman optical tap 37 coupled to the fiber link 28. SNR meter 36 provides anindicator indicative of the signal-to-noise ratio of the optical carrieron output line 38.

The optical fiber 28 terminates at the receive end at an opticalreceiver 40. The optical carrier transmitted through fiber 28 isdetected by a corresponding photo detector 44 generating a signal in theelectrical domain. The output of the optical photo detector 44 isprovided on output line 46 and provided to an electrical splitter 48.Splitter 48 divides the electrical data signal, providing one part to aprimary decision circuit 52 and the other second part to a secondarydecision circuit 54.

The primary decision circuit 52 includes circuitry determining whetheror not the received electrical signal in the time domain is a logic oneor a logic zero. This primary decision circuit 52 is set to an optimalsetting, and has adjustable settings to adjust the decision point as afunction of the circuit design and information from the auxiliarydecision circuitry to minimize the bit error rate (BER). The primarydecision circuit 52 handles the usable data traffic and provides theoutput digital data to a signal tap 58. Digital data is output by tap 58on primary output line 60 to a decoder 62. Tap 58 also provides anidentical stream of digital data via the secondary output line 64 tocomparator 66 as will be discussed shortly. The decoder 62 providesmultiple processing functions to provide, among other things, bit errorcorrection, and also determines a bit error rate (BER). Decoder 62provides a BER signal on output line 72 indicative of the determinedBER. Decoder 62 may be a forward error correction (FEC) decoder as well.The output of the decoder 62 provides the processed digital data fromthe receiver 40 to output line 76. Ideally, the output digital dataprovided on output line 76 is identical to the input electrical digitaldata on input line 26.

Referring now to the secondary auxiliary decision circuit 54, the outputof this circuit 54 is provided on output line 80 to comparator 66.Comparator 66 compares the digital data output of the primary decisioncircuit 52 with the output of the auxiliary decision circuit 54 todetermine differences therebetween. The output of the comparator 66 isprovided on output line 82 to an error counter 84. The auxiliarycircuitry comprising secondary decision circuit 54, comparator 66, anerror counter 84, and a controller 85 is used to ultimately determine aQ factor of the optical carrier and provides this Q factor as anindicator on output line 86 by observing the error count.

Robustness to timewise disturbances is gauged by directing the auxiliarydecision circuit 54 to sample at various time offsets relative to theoptimum set point of the primary decision circuit 52. The secondarydecision circuit 54 has adjustment circuitry which facilitates thedithering of the decision point to different levels to probe the extentsof the operating window. The comparator 66 determines the variance ofthe output from the secondary decision circuit 54 in view of the outputfrom the primary decision circuit 52. Dithering the decision point ofthe secondary decision circuit 54 has no effect on the output digitaldata on output line 76, but allows the decision level to be adjusted todetermine if this improves the Q factor of the optical carrier, andfacilitates adjustments of the optimum setting of the primary decisioncircuit 52 where necessary. In essence, the auxiliary circuitry inreceiver 40 allows the data signal to be analyzed without affecting theprimary receiver circuitry to intelligently determine if improvementscan be made to the primary circuitry.

According to the present invention, controller 22 includes software orequivalent hardware to receive and process various inputs from thevarious portions of the optical circuit 20 to provide fault managementof the optical communications system 20. The controller 22 receives andanalyzes the various inputs, and processes them to distinguish between,among other things, fiber failure, PMD-related degradations, and failureof monitoring equipment itself. The controller 22 provides outputs online 92 that may be used to alter protect switching logic and to alertmaintenance personnel as to the probable cause of degraded pathindications. In the preferred embodiment, four inputs are supplied foreach optical channel to the controller 22 including the bit error rate(BER) observed at the receiver, the Q measurement obtained at thereceiver, alarms or notifications from the PMD compensators along theoptical path, and the optical signal-to-noise ratio as measured by theselective optical power meter tapped onto the path near the receiver.However, limitations to these indicators is not intended and othersystem indicators can be generated and analyzed as well and areencompassed by the present invention. Controller 22 maintains a timestamped record of recent measurements and notifications received via thesupplied inputs.

Referring now to FIG. 4, there is shown a flow diagram of the processingalgorithm of controller 22 according to the preferred embodiment of thepresent invention being generally shown at 100. This method ispreferably implemented in software, but could be implemented in hardwareif desired.

The method starts at step 102 whereby the optical system is initialized.Next, at step 104, the controller 22 determines if degradation of thesystem is observed across multiple optical channels by observingindicators from the system devices associated with these other channels.Collectively, these indicators are received on input line 90 as shown inFIG. 3. If degradation of the system is observed across the multipleoptical channels at step 104, at step 106 the controller 22 will declarea possible fiber failure due to the correlation that several opticalchannels are degraded and degradation is not limited to one channel.

At step 104, if degradation is not observed across multiple channels,the algorithm proceeds to step 108 whereby the current observed BERprovided by decoder 62 via output line 72 is time stamped and recordedat controller 22. Next, at step 110 the controller 22 calculates thepredicted BER based upon the most recent recorded Q measurement providedby error counter 84 via output line 86, as calculated according to thealgorithm 130 of FIG. 5 which will be described shortly. The BER can bepredicted based upon the most recent recorded Q measurement according tovarious known algorithms and mathematical relationships. Some known waysof calculating a BER from the Q measurement are described in the tworeferenced articles identified in the section Background of theInvention entitled “Q-Factor measurements for High Speed OpticalTransmissions”, and “Margin Measurement in Optical Amplifier Systems”,the teachings of which are incorporated herein by reference.

Next, the algorithm proceeds to step 112 where it is determined whetheror not the observed BER recorded in step 108 is significantly betterthan the predicted BER calculated in step 110. If the observed BER isdetermined to be significantly better than the predicted BER, thealgorithm proceeds to step 114 and reports that the most recent recordedQ measurement from step 110 is suspect and false. This report is basedupon the fact that a healthy observed BER is always the reliableindicator of the true operating characteristic of the network 20. If ahealthy BER is being reported by decoder 62, any Q measurement to thecontrary must necessarily be in error.

If at step 112 the observed BER is not determined to be significantlybetter than the predicted BER, the algorithm proceeds to step 116 todetermine if the observed BER is significantly worse than the predictedBER. If the answer is no, the algorithm proceeds back to step 104 sincethe observed BER is generally close to the predicted BER, and thus, themost recent recorded Q measurement is determined to be valid.

If, however, at step 116 the observed BER is determined to besignificantly worse than the predicted BER, the algorithm proceeds tostep 118 to determined whether or not the observed BER has significantlydegraded more recently than the latest recorded Q measurement accordingto the algorithm 130 in FIG. 5, which will be described shortly. If atstep 118 the answer is no, the algorithm proceeds to step 120 andreports that the latest Q measurement recorded in algorithm 130 of FIG.5 is false, or that the receiver 40 is bad. This can be determinedbecause there has been sufficient time for the Q measurement to observethe degrading BER. If at step 118, however, it is determined that theobserved BER has degraded more recently than the latest recorded Qmeasurement provided in algorithm 130, the algorithm has no basis forinvalidating the recorded Q measurement and proceeds back to step 104since a recent degraded BER would account for why the observed BER ismuch worse than the predicted BER based on the most recent recorded Qmeasurement.

In summary, algorithm 100 determines whether or not recorded Qmeasurement is valid or false by determining whether or not thepredicted BER from the recorded Q measurement is in line with therecorded observed BER. Algorithm 100 can also determine whether or notthe receiver 40 is bad.

Referring now to FIG. 5, there is shown generally at 130 a flow diagramof the processing algorithm of controller 22 to record a completed Qmeasurement in a recent history list for use in algorithm 100, at step110.

At step 132, a new Q measurement is determined by error counter 84 andcompleted. This Q measurement typically takes a few minutes to completeas the auxiliary receiver is adjusted to different decision points.Next, at step 134, it is determined whether or not the new Q measurementof step 132 has degraded significantly over the previous Q measurementas provided by error counter 84. If the answer is no, the algorithmproceeds to step 136 to determine if the PMDC 32 is an alarm state. Ifthe answer is yes, the algorithm proceeds to step 138 to report that thePMD alarm indication is false. It is known that the PMD indication isfalse since it is not normal for the PMDC 32 to be in alarm state whenthe Q measurement has not degraded as determined in step 134. If at step136 the PMDC is not in the alarm state, the algorithm proceeds to step140.

At step 140, the controller 22 determines if the optical signal to noiseratio (OSNR) from SNR meter 36 has recently degraded. If the answer isyes, the algorithm proceeds to step 142 and reports a false OSNRindication. It is reported at step 142 that the OSNR indication is falsebecause if the OSNR has degraded, it would have be determined at step134 that the Q measurement would have been degraded.

If, however, at step 140 the OSNR is not determined to have degraded,the algorithm proceeds to step 150 and the new Q measurement of step 132is determined to be valid and is recorded in the most recent historylist at controller 22 for use at step 110 in flow diagram 100.

Referring now back to step 134, if it is determined that the most recentQ measurement of step 132 has appreciably degraded since the last Qmeasurement of the previous iteration of algorithm 130, the algorithmproceeds to step 152.

If at step 152 it is determined that either the PMDC 32 or the SNR meter36 are presently in alarm state by observing the output on line 34 orline 38 from the respective devices, the algorithm proceeds to step 154and reports that the Q measurement degradation determined in step 134 isattributable to the particular PMDC 32 or the SNR meter 36 in an alarmstate. The correlation that the Q measurement is determined to havesignificantly degraded in step 134 in combination with an alarm fromeither the PMDC 32 or the SNR meter 36 is used to isolate whether or notthe Q measurement degradation is attributable to the PMDC 32 or the SNRmeter 36. The respective alarm is indicative of which system device isindicating a problem.

At step 152, typical alarms of the PMDC 32 include, but are not limitedto:

I. the PMD compensator is approaching the limit of its compensatingability;

II. the optical signal has exceeded the compensating range of the PMDcompensator;

III. the range of change of the PMD exceeds at predeterminedcharacteristic value or exceeds a tracking speed of the compensator;

IV. an element in the PMD compensator has failed.

After step 154, the algorithm proceeds to step 150 and records the mostrecent Q measurement in the recent history list as a valid Qmeasurement.

Referring back to step 152, if it is determined that the PMDC 32 and theSNR meter 36 are not in a alarm state, the algorithm proceeds to step156 to determine if the PMDC 32 or the SNR meter 36 were ever in analarm state during the period the Q measurement was taken of step 132.If the answer is yes, the most recent Q measurement is discarded asaberrant and in error since this was a respective system problem asindicated by the PMDC 32 or the SNR meter 36 in alarm state during the Qmeasurement, in step 158. Thereafter, the algorithm proceeds to step 160to complete the processing of the most recent Q measurement of step 132.

If, at step 156 it was not determined that the PMDC 32 or the SNR meter36 were in alarm during the recent Q measurement, the algorithm proceedsto step 162 and reports a possible four wave mixing situation, orpossible failure of the monitoring equipment. Four wave mixing is knownto be able to induce a spurious optical signal which would hurt a Qmeasurement, but which may overlap one of the optical carriers soclosely that it would not degrade the signal to noise ratio asdetermined by the SNR meter 36.

It is noted that degradation in Q factor is usually caused by a poorsignal-to-noise ratio or timing dispersion, or a combination thereof. Ifan abrupt degradation is observed during the course of making Qmeasurements, the notifications from the PMDCs 32 along the line arereviewed by controller 22 to see if a polarization-related anomalyoccurred during that time. This information is used by controller 22 todisregard selected readings taken during the Q measurement, or toinvalidate a Q measurement entirely.

Cross-checking by controller 22 among the Q measurement, SNR, and PMDnotifications also allows for monitoring of the measurement equipmentitself. If it is determined that the Q measurement factor degrades andthe SONET error rate escalates comparably, it is expected that the PMDnotifications and SNR readings would account for the degradation. Ifneither PMD or SNR readings reflect any degradation, then one of thesemonitoring devices is determined by controller 22 to be malfunctioning,or it is determined that an impairment is occurring between the tap formonitoring equipment and the decision circuit of the receiver, perhapsat the receiver front end.

In a different scenario, if a PMD event is registered that would besevere and prolonged enough to theoretically prevent signal reception,yet the SONET BER and Q measurements are not determined by controller 22to degrade appreciably, the PMDC itself may be malfunctioning. Thiscross-checking according to the present invention can preventunnecessary protection switching of optical channels that might resultif only a single indication were relied upon which can disrupt datatraffic.

The method of the present invention integrates and processes severalinput indicators to distinguish between, upon other things, fiberfailure, PMD-related degradations or failure of the monitoring equipmentitself. The outputs provided by controller 22 of the present inventioncan be used to alter protect switching logic and to alert networkmaintenance personal as to the probably cause of degraded pathindications.

The present invention intelligently assimilates and analyzes indicatorsand notifications from various network equipment to provide better faultmanagement in the optical communication system.

Though the invention has been described with respect to a specificpreferred embodiment, many variations and modifications will becomeapparent to those skilled in the art upon reading the presentapplication. For example, anywhere decisions are made upon a degradedmeasurement as in block 140 of FIG. 5, the decision may be based upon afixed threshold or a weighted analysis of recent readings. It istherefore the intention that the appended claims be interpreted asbroadly as possible in view of the prior art to include all suchvariations and modifications.

We claim:
 1. A method of performing fault management in an opticalcommunications system including a polarization mode dispersion (PMD)compensator having the ability to originate notifications indicative ofPMD in the system and an optical receiver, comprising the steps of: a)obtaining a PMD notification from said PMD compensator indicative of PMDin said communications system; b) obtaining at least one indicator froma system device coupled to said optical communications system indicativeof an attribute of said communications system; and c) determining afault of said communications system as a function of said PMDnotification and said system device indicator.
 2. The method asspecified in claim 1 wherein said system device comprises said opticalreceiver and said indicator comprises a Q factor of an optical signalpassing through said communications system.
 3. The method as specifiedin claim 1 wherein said system device comprises said optical receiverand said indicator comprises a bit error rate (BER) of said opticalsignal at said receiver.
 4. The method as specified in claim 2 whereinsaid indicator further comprises a bit error rate (BER) of said opticalsignal at said receiver.
 5. The method as specified in claim 1 furthercomprising the step of obtaining a signal-to-noise ratio (SNR) of saidoptical signal proximate said receiver, wherein said fault determinationof said step c) is a function of said SNR.
 6. The method as specified inclaim 2 further comprising the step of obtaining a signal-to-noise ratio(SNR) of said optical signal proximate said receiver, wherein said faultdetermination of said step c) is a function of said SNR.
 7. The methodas specified in claim 3 further comprising the step of obtaining asignal-to-noise ratio (SNR) of said optical signal proximate saidreceiver, wherein said fault determination of said step c) is a functionof said SNR.
 8. The method as specified in claim 4 further comprisingthe step of obtaining a signal-to-noise ratio (SNR) of said opticalsignal proximate said receiver, wherein said fault determination of saidstep c) is a function of said SNR.
 9. The method as specified in claim 1wherein said PMD notification is selected from the group consisting of:i) the PMD compensator is approaching the limit of its compensatingability; ii) the optical signal has exceeded the compensating range ofthe PMD compensator; iii) the range of change of the PMD exceeds apredetermined characteristic value or exceeds a tracking speed of thecompensator; and iv) an element in the PMD compensator has failed. 10.The method as specified in claim 2 wherein said PMD notification isselected from the group consisting of: i) the PMD compensator isapproaching the limit of its compensating ability; ii) the opticalsignal has exceeded the compensating range of the PMD compensator; iii)the range of change of the PMD exceeds a predetermined characteristicvalue or exceeds a tracking speed of the compensator; and iv) an elementin the PMD compensator has failed.
 11. The method as specified in claim3 wherein said PMD notification is selected from the group consistingof: i) the PMD compensator is approaching the limit of its compensatingability; ii) the optical signal has exceeded the compensating range ofthe PMD compensator; iii) the range of change of the PMD exceeds apredetermined characteristic value or exceeds a tracking speed of thecompensator; and iv) an element in the PMD compensator has failed. 12.The method as specified in claim 4 wherein said PMD notification isselected from the group consisting of: i) the PMD compensator isapproaching the limit of its compensating ability; ii) the opticalsignal has exceeded the compensating range of the PMD compensator; iii)the range of change of the PMD exceeds a predetermined characteristicvalue or exceeds a tracking speed of the compensator; and iv) an elementin the PMD compensator has failed.
 13. The method as specified in claim5 wherein said PMD notification is selected from the group consistingof: i) the PMD compensator is approaching the limit of its compensatingability; ii) the optical signal has exceeded the compensating range ofthe PMD compensator; iii) the range of change of the PMD exceeds apredetermined characteristic value or exceeds a tracking speed of thecompensator; and iv) an element in the PMD compensator has failed. 14.The method as specified in claim 6 wherein said PMD notification isselected from the group consisting of: i) the PMD compensator isapproaching the limit of its compensating ability; ii) the opticalsignal has exceeded the compensating range of the PMD compensator; iii)the range of change of the PMD exceeds a predetermined characteristicvalue or exceeds a tracking speed of the compensator; and iv) an elementin the PMD compensator has failed.
 15. The method as specified in claim7 wherein said PMD notification is selected from the group consistingof: i) the PMD compensator is approaching the limit of its compensatingability; ii) the optical signal has exceeded the compensating range ofthe PMD compensator; iii) the range of change of the PMD exceeds apredetermined characteristic value or exceeds a tracking speed of thecompensator; and iv) an element in the PMD compensator has failed. 16.The method as specified in claim 8 wherein said PMD notification isselected from the group consisting of: i) the PMD compensator isapproaching the limit of its compensating ability; ii) the opticalsignal has exceeded the compensating range of the PMD compensator; iii)the range of change of the PMD exceeds a predetermined characteristicvalue or exceeds a tracking speed of the compensator; and iv) an elementin the PMD compensator has failed.
 17. An optical communications system,comprising: an optical fiber; an optical transmitter coupled to saidoptical fiber generating an optical signal; a polarization modedispersion (PMD) compensator coupled to said optical fiber generating afirst indicator indicative of PMD in said optical fiber; a first systemmeasurement device coupled to said fiber generating a first signalindicative of a condition of the optical signal in said optical fiber;and a controller coupled to said PMD compensator and said measurementdevice determining a fault within said optical communications system asa function of said first indicator and said first signal.
 18. Theoptical communications system as specified in claim 17 wherein saidmeasurement device comprises an optical receiver generating a bit errorrate (BER) of said optical signal as said first signal.
 19. The opticalcommunications system as specified in claim 17 wherein said measurementdevice comprises an optical receiver generating a Q factor of saidoptical signal as said first signal.
 20. The optical communicationssystem as specified in claim 18 wherein said optical receiver furthergenerates a Q factor of said optical signal as a second signal, whereinsaid controller determines a fault within said optical communicationssystem as a function of said first indicator, said first signal, andsaid second signal.
 21. The optical communications system as specifiedin claim 17 further comprising a second measurement device comprising asignal to noise ratio (SNR) detector generating a SNR of said opticalsignal as a second signal, wherein said controller determines said faultwithin said optical communications system as a function of said firstindicator, said first signal, and said second signal.
 22. The opticalcommunications system as specified in claim 18 further comprising asecond measurement device comprising a signal to noise ratio (SNR)detector generating a SNR of said optical signal as a second signal,wherein said controller determines said fault within said opticalcommunications system as a function of said first indicator, said firstsignal, and said second signal.
 23. The optical communications system asspecified in claim 19 further comprising a second measurement devicecomprising a signal to noise ratio (SNR) detector generating a SNR ofsaid optical signal as a second signal, wherein said controllerdetermines said fault within said optical communications system as afunction of said first indicator, said first signal, and said secondsignal.
 24. The optical communications system as specified in claim 20further comprising a second measurement device comprising a signal tonoise ratio (SNR) detector generating a SNR of said optical signal as athird signal, wherein said controller determines said fault within saidoptical communications system as a function of said first indicator,said first signal, said second signal, and said third signal.
 25. Theoptical communications system as specified in claim 17 wherein said PMDcompensator first signal is indicative of a condition selected from thegroup consisting of: i) the PMD compensator is approaching the limit ofits compensating ability; ii) the optical signal has exceeded thecompensating range of the PMD compensator; iii) the range of change ofthe PMD exceeds a predetermined characteristic value or exceeds atracking speed of the compensator; and iv) an element in the PMDcompensator has failed.
 26. The optical communications system asspecified in claim 25 wherein said measurement device comprises anoptical receiver generating a bit error rate (BER) of said opticalsignal as said first signal.
 27. The optical communications system asspecified in claim 25 wherein said measurement device comprises anoptical receiver generating a Q factor of said optical signal as saidfirst signal.
 28. The optical communications system as specified inclaim 25 wherein said optical receiver further generates a Q factor ofsaid optical signal as a second signal wherein said controllerdetermines a fault within said optical communications system as afunction of said first indicator, said first signal, and said secondsignal.
 29. The optical communications system as specified in claim 25further comprising a second measurement device comprising a signal tonoise ratio (SNR) detector generating a SNR of said optical signal as asecond signal, wherein said controller determines said fault within saidoptical communications system as a function of said first indicator,said first signal, and said second signal.
 30. The opticalcommunications system as specified in claim 25 further comprising asecond measurement device comprising a signal to noise ratio (SNR)detector generating a SNR of said optical signal as a third signal,wherein said controller determines said fault within said opticalcommunications system as a function of said first indicator, said firstsignal, said second signal, and said third signal.